Smoking is, with salting, one of the oldest techniques for preserving foodstuffs. Thus, it was discovered shortly after human discovery of fire. At the outset, the object sought was to increase the time of preservation of the treated product. Later, it is principally a matter of the quality of taste and secondarily a way of presenting the product, which prevailed.
Ancient processes have been used up to the last century, but since then, the techniques have been modernized and diversified, the conventional smoking methods representing only a small part of the worldwide volume.
Thus, new products have for example come to light in the United States from the XIXth century. These products, also called liquid smoke or liquid smoke compositions, have been developed to replace the direct contact of the foodstuff with smoke and are obtained by condensing gaseous smokes obtained by pyrolysis of a vegetable organic material, most often wood, in liquid form.
It is thus known that the pyrolysis of vegetable materials, in particular the pyrolysis of particles or chips of wood, leads to the formation of aromatic molecules during the process of thermal decomposition of said vegetable material. The chemical nature of the flavors obtained depends essentially on the treatment parameters, such as the temperature of pyrolysis, the dwell time or else the gaseous atmosphere used in the course of the pyrolysis reaction.
Moreover, most of the chemical compounds constituting the smoke obtained during pyrolysis are liquid at ambient temperature. Because of numerous advantages, these products have tended to constitute progressively the new standard of production of smoked foodstuffs. Thus, the liquid smokes are particularly used advantageously during smoking of ham, sausage, fish, pressed meat etc., a smoked taste as well as a typically brown coloration, similar to those observed during traditional smoking, being thus obtained.
The liquid smokes constitute complex mixtures that can comprise more than 1,000 different chemical compounds, of which 400 have been clearly identified. These compounds appear generally in chemical families whose principal components are carboxylic acids, carbonyls, phenols and polycyclic aromatic hydrocarbons.
By way of example of a typical liquid smoke composition can be cited U.S. Pat. No. 3,106,473.
Schematically, it is noted that the organic acids have an action on the preservability of smoked products, that the phenols have an action on the taste of the smoked products and that the carbonyl compounds give rise to the color of the smoked products. However, because of the extreme chemical complexity of liquid smokes, synergism between the various chemical compounds is more than probable.
A certain number of undesirable compounds are also produced during pyrolysis processes. Polycyclic aromatic hydrocarbons (PAH) are toxic compounds produced during high temperature pyrolysis of organic materials. These compounds must be eliminated from the liquid smokes or their content must at least be minimized. The standards at present in force in Europe require a maximum quantity of 10 ppb of benzo[a]pyrene and 20 ppb of benzoanthracene in liquid smokes.
The control of the content of polycyclic aromatic hydrocarbons in liquid smokes thus permits minimizing the sanitary risks relative to the conventional smoking methods.
For the production of aromatic smokes, various pyrolysis reactors have been developed in the course of these last decades.
In a first type of reactor disclosed in U.S. Pat. No. 4,298,435, the pyrolysis can be carried out in a rotating oven inclined at an angle of 5′. Such an oven is constituted by a rotating calcinor comprising a stainless steel tube that can be brought to the desired temperature. The wood enters the tube at a temperature of 480° such that the only oxygen is provided by the air entrained by the sawing at the time of loading.
In a second type of reactor described in U.S. Pat. No. 3,875,314, the pyrolysis is carried out with the help of a conveyor which passes into a chamber in which there prevails a temperature comprised between 600° C. and 750° C.
In a third type of reactor described in U.S. Pat. No. 4,994,297, an ultra-rapid so-called “flash” pyrolysis (speed of temperature increase of 1000° C./s) permits producing liquid smokes having a carbonyl/phenol ratio greater than the smokes obtained by conventional methods. The smoke which is produced by this type of reactor has a more pronounced coloring power but gives a less intense smoked flavor to the treated food products. The output of pyrolignous juice is very interesting, because it is substantially higher than that obtained by conventional pyrolysis. The sawed wood product or cellulose is heated between 450° C. and 650° C. in one second. The dwell time of the gases emitted is 0.03 seconds to 2 seconds in the reactor, then the gases are evacuated (in less than 0.6 second) so as to cool them to 350° C.
In a fourth type of reactor disclosed in U.S. Pat. No. 4,883,676, the pyrolysis is conducted by ensuring a sweeping of dry air at high temperature over a thin layer (2 cm at the most) of dry sawdust. The output thus reaches 90% instead of 45-50% by conventional methods. The gas produced is very rich in condensable compounds and no tar is produced, said sweeping limiting the secondary reactions which are precisely the origin of the formation of tar. By operating batch-wise, the pyrolysis must be 600° C., whilst when working semicontinuously, the optimum pyrolysis temperature is only 290° C.
The pyrolysis of the wood can also be carried out under steam as described in U.S. Pat. No. 4,359,481, the pyrolysis temperature then being 400° C.
However, most of these installations and processes do not permit strict control of the pyrolysis temperature or the dwell time.
The present invention has for its object to overcome at least certain of the mentioned drawbacks.
To this end, it has for its object a process which relates to a destructive distillation or a thermo-modification of vegetable substances, particularly wood.